Save multiple sessions about tmux-continuum

[Farzat07]

Actually I think continuum does support multiple sessions - what it does not support is multiple servers (using two different sockets for example). This is because tmux-resurrect does save all sessions running on the server when called, and tmux-continuum simply runs that at regular intervals, so I guess tmux-continuum does save all the sessions. Never actually tested that though.

Clarifying this can be nice actually - at least on the docs/multiple_tmux_servers.md page. Some people might get themselves restricted to one session only just because they misunderstood the documentation and thought that only one session is tracked.